This invention relates to a system for extracting information from a carrier wave and relates generally to the method and apparatus described in U.S. Pat. Nos. 4,106,007 and 4,218,655, the disclosures of which are incorporated herein by reference. As described in those patents, it is known that a modulation voltage can be superimposed on a power system voltage, at specified locations on the power system voltage such as a zero crossing, to cause wave shape perturbations in the carrier wave. In the embodiment described hereinafter, the carrier wave is the voltage wave of an electrical power distribution system. Communication over electric power distribution lines is useful for signaling, meter reading, and load control, among other uses. However, communication over an electric distribution system is a complex undertaking. Each customer service constitutes a branch in the distribution feeder, and the branching is so extensive that it is impractical to provide filter and by-pass circuitry at each branch point. The distribution system is not an attractive medium for conventional communications due to the attenuation and dispersion of the signals and because noise levels tend to be high. To overcome the high noise levels, it is generally necessary to use narrow band filtering, error-detecting and error-correcting codes, and relatively high signal power levels at low bit rates.
The aforementioned problems arise in two areas. The first concerns transmitting information from the central source in the direction of energy flow to the individual customer premises. This transmission of information in the direction of energy flow is referred to as "outbound" signaling. Functions such as automatic meter reading and various alarm systems, however, require that information passes not only from a single source to the end user, but also from the end user back to the central station. This transmission of information in the direction opposite to that of the energy flow is referred to herein as "inbound" signaling.
For "outbound" signaling, in order to reach line-to-line customers on the three-phase distribution network of a utility, for example, the modulation signal which carries the information preferably should have dominant positive and negative sequence components. This implies that the outbound modulation signal should not appear on all three phases simultaneously at equal strength and phase relationship.
For inbound signaling, in a grounded neutral four-wire system, when the system is extremely well-balanced, the 60 Hz component of the neutral wire current is zero. The neutral wire current consists basically of third, fifth, seventh, and ninth harmonics only and the additive sum of all distortions of currents in all three phases of the system. Extraction of inbound signals or information in a reasonably well-balanced system where the signal appears line-to-neutral is fairly straightforward and tends to give excellent results. For line-to-line inbound signaling, however, the result is not so favorable. It is known that in those cases where a neutral wire of a three-phase system is not available, one can be synthesized by using current transformers and adding the currents in the three phases to synthesize the neutral. Unfortunately, in line-to-line inbound signaling, the signal current appears on at least two of the phases so that adding together the phase currents by means of current transformers tends to cancel out the signal current.